1. Field of the Invention
This invention relates generally to electronic conditioning circuitry used in conjunction with a capacitive pressure transducer which changes capacitance when subjected to changes in pressure supplied to the sensor, and more particularly to a circuit supplying a high resolution digital output for indicating pressure sensed in a highly accurate fashion, with the electronic circuitry being miniaturized to the greatest extent possible to allow installation into a limited space.
2. History of the Art
The need for highly accurate indications of pressure is particularly keen in the avionics field, where measurement of several different pressures is utilized to provide information about aircraft altitude, mach number, indicated air speed, angle of attack, and slip or yaw angle. It is readily appreciated that on both military and commercial aircraft such information derived from sampled pressures must be highly accurate.
A system for measuring such pressures includes three components, the first of which is apparatus such as a pitot tube for measuring a pressure at a location on the exterior of the aircraft, with a pneumatic line leading from the pitot tube to the interior of the aircraft. The second component of such a system is a transducer for converting the pneumatic pressure into an electrical signal, with the transducer typically being a fused quartz type unit having variable capacitance such as that shown in U.S. Pat. No. 3,962,921, to Lips. The third component of the system is electronic circuitry having as an input the variable capacitance from the quartz sensor. Typical circuitry using an L-C oscillation to produce a frequency which is dependent on pressure sensed by the quartz transducer is shown in U.S. Pat. No. 3,790,910, to McCormack.
It will be readily appreciated that it is desirable to provide a digital output from the electronic circuitry since the output will almost certainly be supplied to an onboard digital air-data computer. The McCormack reference uses a number of counters and registers to condition the signal supplied to a digital computer. More recently, in U.S. Pat. No. 4,208,918, to Miyamae, pressure sensor circuitry is shown for providing a digital signal to a counter which then will drive a display through a decoder/driver.
Systems such as those previously described operate quite well and have been commercially successful in the past. However, with the increase in the amount of avionic equipment carried by aircraft in combination with the desire to minimize space requirements and weight of such equipment it is desirable to package the sensor and accompanying electronic circuitry in the smallest possible package. The main limitation on decreasing the size of the fused quartz capacitive sensor and accompanying circuitry is dictated by the design of the circuitry, which is an L-C circuit. As is evident by an examination of the Lips sensor, the inclusion of an inductor is responsible for between 80-90% of the size (and weight) of the sensor itself, since the inductor is typically built into the sensor. It may therefore be appreciated that any circuitry utilizing L-C resonance will be unacceptably large, and it is accordingly evident that a different design in the electronic circuitry is required.
The miniaturized design must retain at least as high a degree of precision as the older L-C design. In addition, it must have good resistance to noise, a high degree of resolution, and long term stability and dependability. The sensor circuitry must also be unsusceptible to variations in stray capacitance emanating from the quartz sensor itself, a common problem. It may therefore be appreciated that a miniature electronic circuit for converting the variable capacitance signal from a quartz transducer into a digitally readable electronic signal having high precision is a highly desirable product.